Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than traditionally used silicon, germanium, or gallium arsenide. Diamond provides a higher energy band gap, a higher breakdown voltage and a greater saturation velocity than these traditional semiconductor materials. These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using conventional semiconductor materials. For example, silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 Ev at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments. In other words, diamond is also a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices. Key to many of such devices, such as diodes and field effect transistors (FET's), is an ohmic contact having good electrical and mechanical characteristics even at relatively high operating temperatures. Consequently, the fabrication of ohmic contacts on diamond will play an important role in the development of future diamond-based semiconductor devices.
Ohmic contacts have reportedly been obtained on semiconducting diamond. For example, U.S. Pat. No. 5,055,424, to Zeidler et al. discloses a refractory metal layer forming an ohmic contact on semiconducting diamond. The patent to Zeidler et al. also discloses a refractory metal carbide interface region between the diamond layer and the refractory metal layer formed by heating the structure. Similarly, Moazed et al. in A Thermally Activated Solid State Reaction Process for Fabricating Ohmic Contacts to Semiconducting Diamond, J. App. Phys., 68(5), Sept. 1990, discloses annealing a refractory metal, that is, molybdenum, at 950.degree. C. to grow carbide precipitates at an original diamond/metal interface to provide an electrical contact with good electrical performance and with good mechanical adhesion at relatively high operating temperatures.
The electrical resistivity of ohmic contacts to diamond has also been improved by highly doping the surface region of the diamond underlying a metal contact as disclosed, for example, by Tsai et. al., in Diamond MESFET Using Ultrashallow RTP Boron Doping, IEEE, Electron Dev. Letters., Vol. 12, No. 4, Apr. 1991. The Tsai et al. article discloses using cubic boron nitride in a solid state diffusion process including rapid thermal annealing for more highly doping the surface portion of the diamond layer. An article to Prins entitled Preparation of Ohmic Contacts to Semiconducting Diamond, pp. 1562-1564, July 1989, discloses ion implantation to more highly boron dope a surface of diamond underlying a metal contact layer to lower resistivity.
Another approach to obtaining a highly doped diamond surface portion adjacent a metal contact layer is disclosed in an article by Grot et al. in The Effect of Surface Treatment on the Electrical Properties of Metal Contacts to Boron-Doped Homoepitaxial Diamond Film, IEEE Electron Dev. Letters. Vol. 11, No. 2, Feb. 1990. The article discloses placing boron powder near edges of a diamond substrate and then exposing the powder and the diamond to a hydrogen plasma to thereby highly dopes the diamond surface portion with boron.
Ishii et al., in U.S. Pat. No. 5,075,757 teaches a non-metal ohmic contact formed by a highly boron-doped silicon layer on a diamond layer, wherein the silicon is amorphous or polycrystalline including microcrystalline silicon phase. The diamond region underlying the silicon layer becomes more highly doped by boron diffusion from the silicon layer into the diamond surface.
Despite continuing attempts to obtain low resistivity ohmic contacts to diamond also having good mechanical adhesion even at elevated temperatures, there still exists a need for such contacts which may be readily fabricated.